Method of manufacturing core-shell particles by a microwave plasma process

ABSTRACT

Methods are disclosed for producing core-shell particles having a uniform size using a microwave plasma process. More particularly, methods of the present technology are used to manufacture core-shell particles having a core at least partially surrounded by a shell. The core and shell of the core-shell particles are chemically distinct. Methods of the present technology occur within a plasma chamber of a microwave plasma reactor and a microwave formed plasma is utilized to vaporize core precursor material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/716,809 filed Dec. 17, 2019 (issued as U.S. Pat. No. 11,311,937 onApr. 26, 2022) which is a divisional of U.S. patent application Ser. No.15/347,225 filed Nov. 9, 2016 (issued as U.S. Pat. No. 10,543,534 onJan. 28, 2020). The entire disclosures of the aforementioned patentapplications are incorporated herein by this reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to systems, methods, and devices forproducing quantum particles (e.g., quantum dots). More particularly, thepresent disclosure relates to systems, methods, and devices forproducing quantum particles having a uniform size by vaporization ofdroplets formed from a melt of solid materials (i.e., a melt made fromheating solid material(s) to a temperature above its meltingtemperature).

BACKGROUND OF THE DISCLOSURE

Quantum dots (QDs) are of interest in a number of fields, includingelectronics, photovoltaics, computing, imaging, catalysis, and medicine.Quantum dots are a form of semiconductor nanoparticles having a smallsize on the order of about 1 to about 100 nm, or from about 1 to about10 nm, and which may have optical, electronic, or magnetic propertieswhich vary from those of bulk materials. In particular, many of theseproperties may vary with the size of individual dots. That is, one ormore material properties can be altered or changed by merely a change insize of the dot through the angstrom and nanometer scales. For example,in general conductivity of metals typically goes down as temperaturegoes up. The rate of change of conductivity with temperature may bealtered by size changes in the nanometer scale. As a result, tightcontrol over size is an important factor in the manufacture of quantumdots. These properties may vary with the size of the particles either ina linear or continuous proportion with size changes, or in a quantizedfashion, that is the value associated with a given property may changein step-wise manner at particular sizes of the particle, but will berelatively constant in-between those steps.

Other factors that can affect the quality or commercial manufacture ofquantum dots include purity and stoichiometry. For example, the lack ofcontrol over purity, such as by the presence of contaminants mayinterfere with the formation of the desired crystalline structure orresult in broadening of the electronic and optical characteristic of theparticles.

Stoichiometry of quantum dots is an important consideration. Theparticular stoichiometry may control the ability of particles to form atall, as well as characteristics (electrical, optical, magnetic, etc.) ofthe quantum dot. Some quantum dots may have two or more components whichare each present in substantially comparable concentrations, e.g. 1:1,2:1, 3:1, 4:1, 5:1, 10:1. Other quantum dots may have one or moredopants which are presents in much smaller quantities, such as 100:1,500:1, 1000:1. Quantum dots may also have both one or morestoichiometric components and one or more dopants.

Precise control of particle size may also be critical to manyapplications. The electrical and optical characteristic of any givenparticle may depend on its size, whether in a quantized or proportionalrelationship. In addition to the size of individual particles, thedistribution of size within a population of the same quantum dot mayalso be important. In many applications, a narrow size distribution willbe desired so that characteristics that correspond to size will also berelatively narrow, e.g., narrow absorbance and emission bands. Sizeeffects may be proportional within certain size domains and may bequantized within certain other size domains.

Existing synthetic methods for forming quantum dots may largely begrouped as physical and chemical methods. Physical methods include inertgas condensation, arc discharge, ion sputtering, laser ablation, andpyrolysis. Chemical methods may be grouped under various categories,such as by solvent used (aqueous or organic) or conditions of thereaction (e.g., solvothermal reactions carried out above the normalboiling point of the solvent by applying high pressure, or arrestedprecipitation reactions at high pH).

Conventional synthetic methods are plagued with multiple problems. Forexample, chemical methods may utilize hazardous or expensive solventprecursors. In addition, disposal of waste solvents can be expensive.Further, efficient conversion of precursor material plagues bothsynthetic method routes. That is, typically a large amount of waste isgenerated in processing the precursor materials in comparison to theamount of quantum dots manufactured.

Another measure of efficiency relates to control over particle size.Where particle size is widely distributed, it may be necessary toseparate the desired product particles and discard particles outside theappropriate tolerance. This results in a further reduction isefficiency.

SUMMARY OF THE TECHNOLOGY

The current technology relates to a novel means for manufacturingquantum particles (e.g., quantum dots), through the use of a dropletmaker-plasma chamber system. The system can achieve one or more of highpurity, a narrow size distribution, and/or desirable electronic andoptical characteristics in the product quantum dots. Furthermore, thesystem can operate at lower cost, more rapidly, and/or with greaterenvironmental sensitivity, as compared to existing methods.

In an embodiment, the present disclosure relates to a system for themanufacture of quantum dots from a solid precursor material (i.e., froma molten melt made from heating solid material at or above its meltingtemperature). The system includes a molten material while maintainingthe droplets in a molten state, the molten material being formed fromthe solid precursor material; a plasma chamber in communication with thedroplet maker; at least one inlet for introducing a gas or vapor intothe plasma chamber; an energy source (e.g., a microwave energy source)for forming the plasma, the energy source in communication with theplasma chamber and disposed to apply energy to the gas or vapor in theplasma chamber to form the plasma at a location within the plasmachamber to permit the flow of molten droplets to be vaporized to form aquantum particle precursor material; and a quenching chamber incommunication with the plasma chamber and disposed to receive thequantum particle precursor material, the quenching chamber adapted toquench the quantum particle precursor material to inhibit growth and toform the quantum particles.

In an embodiment, of the above system for the manufacture of quantumdots from a solid precursor material requires that the substantiallyuniformly sized droplets of molten material each has a droplet diameterthat is within 50% (e.g., 25%, 10%, 5%) of a predetermined droplet size.In another embodiment, the present disclosure relates to a system forthe manufacture of quantum dots from a solid precursor material (e.g.,molten melt of the solid material) where the energy source includes amicrowave energy source. In an embodiment, the present disclosurerelates to a system for the manufacture of quantum dots from a solidprecursor material where the microwave energy source has a radiationfrequency from about 900 MHz to about 5900 MHz. In an embodiment, thepresent disclosure relates to a system for the manufacture of quantumdots from a solid precursor material where the droplet maker includes aheating element disposed to maintain the molten material formed from thesolid precursor material in the molten state. In an embodiment, thepresent disclosure relates to a system for the manufacture of quantumdots from a solid precursor material includes a piezo electric actuator,a flexible plate, and at least one capillary for delivery moltenmaterial. In an embodiment, the present disclosure relates to a systemfor the manufacture of quantum dots from a solid precursor materialincluding a low-pressure pump in communication with the quenchingchamber and disposed to produce low pressure within the quenchingchamber. In an embodiment, the present disclosure relates to a systemfor the manufacture of quantum dots from a solid precursor materialwhere the quenching chamber comprises one or more exterior walls and acooling fluid source, at least one exterior wall comprising a pluralityof holes, the plurality of holes disposed to receive a flow of coolingfluid from the cooling fluid source.

In an embodiment, the present disclosure relates to a method for themanufacture of quantum particles (e.g., quantum dots) from a solidprecursor material. The methods includes melting the solid precursormaterial to create a molten precursor material; forming droplets of themolten precursor material; vaporing the droplets within a plasma to forma plasma treated vapor; and quenching the plasma treated vapor to formquantum particles.

In an embodiment, the present disclosure relates to a method for themanufacture of quantum dots from a solid precursor material includingintroducing a second vapor including functionalizing material. Thesecond vapor can be added prior to the quenching step, during thequenching step, r after the quenching step. In an embodiment, thepresent disclosure relates to a method for the manufacture of quantumdots from a solid precursor material including introducing a secondvapor prior to the quenching step, the second vapor including anallowing material to form a mixture with the plasma treated vapor. In anembodiment, the present disclosure relates to a method for themanufacture of quantum dots from a solid precursor material where thesecond vapor including silane for forming silicon based alloys. In anembodiment, the present disclosure relates to a method for themanufacture of quantum dots from a solid precursor material includinggenerating the plasma, which includes generating microwave radiation,guiding microwave radiation to the plasma chamber using a waveguide,introducing an entrainment gas into the plasma chamber, and igniting theplasma. In an embodiment, the present disclosure relates to a method forthe manufacture of quantum dots from a solid precursor materialincluding mixing the molten precursor material prior to formingdroplets. In an embodiment, the present disclosure relates to a methodfor the manufacture of quantum dots from a solid precursor materialwhere the quenching step occurs partially in a plasma chamber andpartially in a quenching chamber. In another embodiment, the presentdisclosure relates to a method for the manufacture of quantum dots froma solid precursor material where the vaporizing step occurs in a plasmachamber and the quenching step occurs in a quenching chamber, and apressure gradient is established and such that the quenching chamber ismaintained at a lower pressure than the plasma chamber. In anembodiment, the present disclosure relates to a method for themanufacture of quantum dots from a solid precursor material includingmaintaining the plasma chamber between about 700 to 1100 Torr andmaintaining the quenching chamber between about 1 to 100 Torr. In anembodiment, the present disclosure relates to a method for themanufacture of quantum dots from a solid precursor material where thestep of forming droplets comprises agitating the molten precursormaterial with a piezoelectric actuator and forming the molten precursormaterial through at least one capillary. In an embodiment, the presentdisclosure relates to a method for the manufacture of quantum dots froma solid precursor material where the quenching step is achieved bycausing super saturation, nucleation and controlled growth (e.g.,limited growth) to form the quantum particles/dots. In an embodiment,the present disclosure relates to a method for the manufacture ofquantum dots from a solid precursor material where the quenching stepincludes establishing low pressure in the quenching chamber, the lowpressure being sufficient to achieve rapid expansion and condensation ofthe vapor. In an embodiment, the present disclosure relates to a methodfor the manufacture of quantum dots from a solid precursor materialwhere at least 50% of the precursor material consumed in the method isconverted into quantum dots. In an embodiment, the present disclosurerelates to a method for the manufacture of quantum dots from a solidprecursor material where the quenching step comprises creating acontrolled gaseous environment in which the plasma treated vapor passesthrough. In an embodiment, the present disclosure relates to a methodfor the manufacture of quantum dots from a solid precursor materialwhere the controlled gaseous environment includes one or more elementalgases. In an embodiment, the present disclosure relates to a method forthe manufacture of quantum dots from a solid precursor material wherethe controlled gaseous environment includes one or more molecular gases.In an embodiment, the present disclosure relates to a method for themanufacture of quantum dots from a solid precursor material where thequenching step includes flowing a cooling fluid proximate the plasmatreated vapor. In an embodiment, the present disclosure relates to amethod for the manufacture of quantum dots from a solid precursormaterial including a functionalizing step, which includes providing afunctionalizing gas or vapor within the quenching chamber where thefunctionalizing gas or vapor contains a function group precursor. In anembodiment, the present disclosure relates to a method for themanufacture of quantum dots from a solid precursor material, wherein thesolid precursor material is a material selected from the groupconsisting of: tin, selenium, aluminum, gallium, indium, cadmium, zinc,lead, bismuth, europium, arsenic, iodine, thallium, silver, strontium,lithium, barium, and alloys of the same. In some embodiments, the solidprecursor material is any element or alloy with a melting temperaturelower than about 1000 C. In general these solid precursor materials canbe used to form a semiconductive material. In one embodiment, the methodof the present disclosure further includes a filtering or collectionstep.

In an embodiment, the present disclosure relates to a method ofmanufacturing a quantum dot having a core at least partially surroundedby a shell. The method includes: melting a core precursor material tocreate a molten core precursor material; forming core precursor materialdroplets from the molten core precursor material; vaporizing the coreprecursor material droplets within plasma; forming quantum dot coresfrom the vaporized core precursor material droplets; melting a shellprecursor material to create a molten shell precursor material; formingshell precursor material droplets of the molten shell precursormaterial; vaporizing shell precursor material droplets within plasma;and forming quantum dot shells on the quantum dot cores.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features provided by embodiments of the presenttechnology will be more fully understood from the following descriptionwhen read together with the accompanying drawings.

FIG. 1 shows a flow chart of a method for making quantum particles(e.g., quantum dots) in accordance with an embodiment of the presentdisclosure;

FIG. 2 shows a schematic view of a system for making quantum dotsaccording to the present disclosure;

FIG. 3 shows a schematic view of a plasma torch suitable for use in anembodiment of the present disclosure; and

FIG. 4 shows a schematic view of a system for making quantum dots havingtwo plasma torches according to the present disclosure.

DETAILED DESCRIPTION OF THE TECHNOLOGY

Embodiments of the present disclosure may produce quantum particles,such as for example, quantum dots, from a substantially pure solidmaterial feedstock with a high degree of efficiency of conversation andwherein a given population of particles are substantially uniformlysized. These may be highly valuable properties for quantum dots. Inembodiments, the substantially pure solid material feedstock hasrelatively low melting temperature. That is, in some embodiments, themelting temperature of the solid precursor is less than about 2,000degrees F. In certain embodiments, the melting temperature of the solidprecursor is less than about 1,450 degrees F. In certain embodiments,the melting temperature of the solid precursor materials is within arange of 800 degrees F. to about 82 degrees F. That is, the meltingtemperature of the solid precursor materials, in some embodiments, isabove room temperature but below 2,000 degrees F.

With respect to the feedstock, the ability to use a substantially puresolid material feedstock presents an advantage over existing chemicalquantum dot synthesis. In wet chemical synthesis of quantum dots, harshand environmentally unfriendly solvents detract from achievingcommercial sustainability. For example, in making some quantum dots,liquid organometallic material is typically the source of precursormaterial. Use of an organometallic precursor may add cost andcomplexity, since the precursor itself must be purchased or synthesized.It may also reduce the yield of the quantum dots (QDs) syntheticprocess, since a substantial portion of the reaction mixture results inthe formation of an organic moiety, and not a quantum dot. Aftersynthesis of the QD, some byproduct of the synthesis related to theorganic moiety will be formed which must be disposed. Some such organiccompounds may be toxic. Additionally, the presence of portions ofreacted and unreacted organic substances within the solution may reducethe purity and uniformity of the finished product.

In addition to the above disadvantages, wet chemistry solutions cannotbe used to synthesize certain materials. For example, nitride quantumdots generally cannot be synthesized with wet chemistry and are oftendifficult to manufacture due to complications including materialstability, such as low decomposition temperatures, difficulties inlimiting oxide and diatomic nitride formation, nitrogen deficiency andparticle agglomeration. In methods using conventional systems to processnitride quantum dots, high temperature solutions such as metal oxidechemical vapor deposition (MOVCD) or molecular beam epitaxy (MBE) hasbeen utilized. These manufacturing routes suffer from slowdeposition/production rates, making this route commercially unfeasible.In addition, substrate materials are expensive and processing conditionsrequire frequent cleaning and monitoring of the deposition environment.Other methods used in place of wet chemistry include, laser ablation ofsubstrate materials. Laser ablation techniques involve ablating a targetin a controlled atmosphere to deposit a material on a substrate. LikeMOVCD and MBE, laser ablation techniques suffer from a low productionrate and high cost of manufacture. In addition to these problems, it ishard to control deposition size and size distribution as a laser sparkis used to ablate the material that then transfers to a cooler substratelocated within the deposition chamber. As a result of its inability tohave a tight control over size and size distribution, laser ablation isa poor choice for the manufacture of quantum dots.

An advantage of one or more embodiments of the present technology is anability to convert solid precursor materials to QDs with a high percentyield (that is, actual yield/theoretical yield). In theory, this yieldcan be as high as 100%, as all the solid precursor material cantheoretically be transformed into a quantum particle product. But inpractice there may be some material lost to the walls of the plasmachamber or system that will reduce the yield slightly from 100% (e.g.,99.5%, 99%, 98.5%; 98%, 97%, 95%). As a result of the high percentageyield, embodiments of the present technology are well suited forcommercialization. In particular, QDs or other quantum particles whichcontain particularly expensive precursor materials, can be manufacturedin large quantities at a relatively low cost to provide for an availablecommercial route for quantum dot manufacture.

Another advantage of embodiments hereof is the ability to achieve auniform particle size. This advantage allows the current technology tosuccessfully and reproducibly manufacture a quantum dot and anappropriate size distribution for a population of quantum dots of agiven type. In some embodiments, the devices and methods describedherein provide uniform conditions for successive quantum dot particlesover an extended period that a high degree of uniformity within thequantum dot particles may be achieved. The present technology may beable to achieve a considerably larger quantity of product within a givenrun than by other systems. Existing synthetic methods, especiallychemical methods, may rely upon a batch-type manufacturing process inwhich the actual amount of quantum dot forming material within eachbatch may be relatively small compared to reactive liquid and otherchemicals which result in by-products or waste. One such exampleincludes forming a quantum dot by wet chemical methods. Specifically,the organometallic precursor liquid compounds used in a reaction to formthe quantum dots have a minimal concentration of metal ions as comparedto the other constituents of the organometallic precursors. In addition,such methods may not scale well for manufacture of more product quantumdots, as the vessels for handling the reactions and devices needed toextract waste become unruly.

The present technology may achieve an increased output for severalreasons, including, the use of a substantially pure solid precursormaterials, such that substantially all of the material may be used withthe quantum dots; the ability to scale up the molten material meltchamber to hold a larger quantity of molten material without requiringthe scale-up of other system components, unless desired; the ability insome systems to add additional solid material feedstock to the moltenmetal melt chamber during operation to extend the period of operation,or to provide for more than one molten metal melt chamber in order toincrease the capacity of the system and permit one chamber to berefilled while another continues to provide supply.

With respect to the materials which can be used herein as solidprecursors, some materials may be more or less suitable for use in thepresent technology based upon factors such as melting point, viscosityof the molten material and so on. Additionally, toxicity and stabilitymay require operating the present technology with containment means tocontrol e.g., gaseous portions of the product or airborne particulates.Embodiments of the present technology are anticipated for melting pointsup to about 1,000° C. (less than 2,000° F.). In certain embodiments, themelting point of the precursor will be much less than 1,000° C., andpreferably within a range of 700° C. to 27° C. Additionally, it may beappreciated that the present technology may be coupled with other knowntechnologies to achieve quantum dots in which one or more pure materialcomponents would melt at too high a temperature for the currenttechnology. For example, silicon, which melts at about 1,414° C., may beadded to the system as silane vapor (SiH₄), to be coupled with a secondmaterial (i.e., having a melting temperature less than about 1,000° C.)prepared using the present technology to create a quantum dot includingboth the 1,000° C. or less melting point material and silicon (e.g.,tin-silicon quantum dot).

A non-limiting list of exemplary materials which can be used as thesolid precursor materials includes: tin, selenium, aluminum, gallium,indium, cadmium, zinc, lead, bismuth, europium, arsenic, iodine,thallium, silver, strontium lithium, barium, and alloys of the same. Itis also possible to create alloys or mixtures by using solid particlesof two different materials, heating the two materials and mixing themolten materials together. The materials may more particularly includemetals and metalloids, but are not limited to metals and metalloids.

An embodiment of a method 10 to manufacture quantum dots is provided inFIG. 1 . The method includes preparing a molten precursor material fromsolid feedstock (step 12) (e.g., heating solid feedstock to atemperature at or above its melting temperature to create a melt fromthe solid feedstock); producing a flow of uniformly sized droplets ofmolten precursor material (step 14); transferring flow of uniformlysized droplets of molten precursor material to a plasma chamber (step16); providing gas or vapor to the plasma chamber with the moltendroplets (step 18); generating a plasma in the chamber to vaporize theprecursor material (step 20) (e.g., directing energy to the plasmachamber to ignite the vapor/gas forming the plasma); and quenching thevaporized precursor material to form the quantum dots (step 22). Toperform method 10 requires a system which includes components capable ofor designed to: (1) make a molten material from solid feedstock (e.g., aheater); (2) make a flow of uniformly sized molten droplets (e.g., adroplet maker that can control size while maintaining the molten stateof the flow); (3) make a plasma (e.g., a plasma torch and plasma chamberin communication with the flow of droplets as well as at least onesource of gas or vapor); and (4) collect the quantum dots from theplasma chamber (e.g., a collection unit which can include a quenchingchamber).

In general, the heater can be any heater capable of forming a moltenmaterial from solid feedstock. In order to create droplets of the solidprecursor materials themselves-without requiring the use of a liquidprecursor (e.g., an organometallic precursor), the use of a heater orother thermal energy conduction device is necessary to melt the solidprecursor materials. The heater may be any type of heater suitable forincreasing the temperature within a desired range, such as an electricalresistor, a heat pump, infrared heater (e.g., quartz tungsten heater),microwave heater, induction heater, or laser heater.

In some embodiments the heater and the droplet maker are separatecomponents. That is, the heater forms a molten material from the solidfeedstock in a molten melt chamber and delivers the molten material to adroplet maker capable of producing uniformly sized droplets of a moltenflow. In other embodiments; the heater and the droplet maker areintegrated such that the molten material is produced within a reservoirof the droplet maker. The necessity for a heater within the dropletmaker may be determined by a number of factors, such as the meltingpoint of the solid precursor material, the thermal conductivity of theprecursor material, and the degree to which the components of the systemare insulated. For example, in an embodiment wherein the melting pointis relatively low, thermal conductivity is relatively low, andinsulation is relatively high, the necessity for additional heating willbe reduced because the molten material will more easily remain a liquidas it travels from the molten melt chamber to the droplet generator. Inany embodiment, the heater and droplet maker of a system of the presentdisclosure must be able to transform solid feedstock into a moltenstate, create a flow of uniformly sized droplets from the moltenmaterial, and maintain the molten state (i.e., substantially maintainthe molten state) of the uniformly sized droplets when delivering flowto the plasma chamber for the creation of quantum dots. Theserequirements dictate that the droplet maker and its components usedcontrol and tune size of the droplets be equipped to handle moltenmaterials. That is, the components needed for generation of the dropletsbe able to withstand temperatures and other conditions of the moltenmaterial. Further the droplet maker needs to be able to not only controlthe size of each droplet within in a flow to create uniformly sizeddroplets, but also to be able to maintain this control even afterexposure to the molten material during a manufacturing run. In general,uniformity means that the droplets have a 10% diameter size variation orless. In some embodiments, the variation is within 5% between droplets(e.g., 5%, 4%, 3%, 2%, 1%, 0.5%). In some embodiments, the droplet makerneeds to be tunable. That is, the droplet maker, in some embodiments,allows an operator to change or select the size of the droplets. Forexample, in a first manufacturing run, an operator selects a dropletsize of 80 micrometers; while in a second manufacturing run, theoperator selects a droplet size of 10 micrometers. In all embodiments,the droplet maker needs to be capable of producing micrometer orsub-micrometer sized droplets. In general, the micrometer tosub-micrometer range includes 0.1 micrometers to about 250 micrometers.

An example of a droplet maker which is capable of providing a flow ofuniformly sized droplets with tunable size control is provided in U.S.Pat. No. 9,321,071, and entitled “High frequency uniform droplet makerand method” (incorporated herein by reference in its entirety). Further,molten droplet makers are known and can be used as a component of thepresent disclosure. For example, U.S. Pat. No. 9,307,625, entitled“Droplet dispensing device and light source comprising such a dropletdispensing device” (incorporated herein by reference in its entirety)discloses a device with improved precision of droplet size andtrajectory even when the liquid used is molten material from a heatedreservoir. US 2005/0253905, entitled “Droplet generation by transversedisturbances” (incorporated herein by reference in its entirety)discloses a droplet maker that enables the formation of molten dropletsdue to capillary stream break-up and minimizes variation in dropletformation time by applying transverse disturbance to initiateinstability of the capillary stream's surface. Other known moltendroplet makers are disclosed in U.S. Pat. No. 5,598,200 and WO 96/22884(PCT/US96/01 132) both entitled “Method and apparatus for producing adiscrete droplet of high temperature liquid”, and “Molten AluminumMicro-Droplet Formation and Deposition for Advanced ManufacturingApplications” by Orme et al and accepted for publication in AluminumTransactions Journal, 2000, all three disclosures are incorporatedherein in their entirety.

The droplet diameters preferred for the present technology may be on theorder of 1 to 200 microns. It is advantageous to form small droplets(e.g., on the range of 1 to 25 microns) that are substantially uniformin size (e.g., each of the droplets has a size that is within 50%, 30%,25%, 10%, 8%, 5% from a predetermined droplet size). Without wishing tobe bound by theory, it is believed that substantial uniformity ofdroplet size combined together with a small droplet size (e.g., lessthan 200 microns, less than 100 micron, less than 50 microns, less than25 microns) allows for consistent and uniform evaporation of thedroplets when subjected to a plasma. Uniformity and consistency invaporization of the droplets create an advantageous environment fornucleation, crystallization, and initiation of growth of a quantumparticle. As a result, in certain embodiments of the present technology,the droplet makers are capable of forming a stream of substantiallyuniform droplets, each droplet having a droplet size of about 50 microns(±25%).

In general, the plasma torch may be any plasma torch capable of forminga plasma by the supplying microwave radiation to a gas. The dropletsformed by the droplet maker may then be introduced to and processedwithin the plasma formed by the plasma torch. Examples of plasma torchesmay be found in U.S. Pat. No. 8,748,785 entitled “Microwave PlasmaApparatus and Method for Materials Processing,” U.S. Pat. No. 8,951,496entitled “Method for Making Amorphous Particles Using a UniformMelt-State in a Microwave Generated Plasma Torch,” U.S. Pat. No.9,242,224 entitled “Method for the Production of Multiphase CompositeMaterials Using Microwave Plasma Process,” and in US Publication No.2013/0270261 entitled “Microwave Plasma Torch Generating Laminar Flowfor Materials Processing,” (each of the foregoing incorporated in fullherein by reference).

The collection of the quantum dots may be accomplished by a collectorunit. The collector unit may be in communication with the quenchingchamber or both may be one component. The collection process may bedesigned to avoid or reduce agglomeration of the quantum dots,particularly for those quantum dots prone to agglomeration. For example,the quantum dots may be collected in filters as described in USPublication No. 2015/0126335 entitled “Method for Making AmorphousParticles Using a Uniform Melt-State in a Microwave Generated PlasmaTorch.” The collector unit may provide for further processing of thequantum dots such as filtering, sorting, cleaning, packing, etc.

The quantum dots made with the technology described herein can includevarious features. For example, the quantum dots may be made of a singlematerial or of more than one material. Where the quantum dot is madewith more than one material, the two materials may be mixed throughoutthe structure of the quantum dot (two or more solid precursors can bemelted and mixed to create a homogeneous melt), or may be in acore-shell arrangement, where one material or mixture of materials formsa core while a second material or mixture of materials forms a shellcovering the core. Further, the quantum dots may additionally have anouter functionalizing layer, which may be an organic moiety. Thefunctionalizing layer may impart properties, such as decreasingagglomeration of the quantum dots, providing linkers through which thequantum dot may be bonded to, e.g., a substrate, or adding complementaryelectronic or optical properties (e.g., organic chromophores suitablefor quenching quantum dot emissions).

An embodiment of a melt chamber is shown in FIG. 2 , which shows system100 having molten melt chamber 101. Molten melt chamber 101 includes abody 104 which is surrounded by insulation 102. The upper portion ofbody 104 is occupied by a layer of pressurized gas 103. The remainder ofthe body may be occupied by the precursor material, in either solidform, or in molten form if a heat treatment has been applied. Encirclingbody 104 is heating element 105. Heating element 105 is depicted as aheating element which spools around a portion of body 104 however theheating element may be some other type of heating element suitable formelting and maintaining the solid material precursor within body 104 ina liquid/molten phase. Heating element 105 provides the thermal energyused to melt the solid material within the molten melt chamber 101.Molten melt chamber 101 is additionally provided with a gas supply 108and a vent 109. Gas supply 108 and vent 109 are in communication withthe layer of pressurized gas 103 and may increase or decrease thepressure within the layer of pressurized gas 103 by increasing ordecreasing the amount of gas present. The gas supply 108 may also becapable of substituting one gas for another within the layer ofpressurized gas 103, if desired, or achieving a purge of the moltenmetal melt chamber 101 as described below. Molten transport line 106 hasone end point in fluid communication with the molten melt chamber 101 totransport molten material from the molten melt chamber 101 to dropletmaker 110.

In operation, the molten melt chamber may be operated by loading thechamber with the amount of solid material feedstock which is desired tobe used. Then, the melt chamber temperature may be increased to melt thefeedstock. In order to provide sufficient purity within the precursorsupply, the melt chamber may be purged with a gas, such as an inert gasand then that gas then drawn off. This may remove species introducedfrom the atmosphere, particularly including atmospheric oxygen, whichmay be reactive, as well as contaminants which have been released duringthe melting of the solid precursor material, which may have evaporated,pyrolyzed, etc. The purge gas may be an inert gas, such as argon. In anembodiment, a similar result may be achieved by drawing a vacuum withinthe molten melt chamber such that any contaminants or trace componentsmay be removed. In an embodiment, a mechanical stirrer maybe added tothe melt chamber to facilitate melting and/or mixing of one or moreprecursor materials and to release contaminants present within thechamber after melting of the material.

Once the molten metal has been satisfactorily prepared and, in someembodiments, purified, a pressure of gas may be applied to the moltenmetal melt chamber. The pressure of gas may be about 5 to about 100Torr. The pressure may be varied, for example, to change the rate atwhich the molten material is supplied. The pressure may be about 5 Torr,about 10 Torr, about 20 Torr, about 30 Torr, about 40 Torr, about 50Torr, about 60 Torr, about 70 Torr, about 80 Torr, about 90 Torr, orabout 100 Torr. These values may also be used to define a range, such asfrom about 30 Torr to about 80 Torr. Preferably, this gas willsubstantially form a separate layer over the molten material, which maythen exert a force there against, tending to push the melt into moltentransport line 106. The gas chosen should generally be selected forminimum reactivity with the metal present in the molten metal meltchamber. In some cases, air may be used, or an inert or relativity inertgas, such as N₂, Ne, Ar, Kr, Xe, etc.

Other embodiments of molten melt chamber may use different components toprovide comparable functionality. For example, the force used to achievethe transfer of the molten material may be some other means than theaction of the layer of pressurized gas 103, for example a pump, such asa peristaltic pump or screw pump.

The combination of the heater with the materials used for the moltenmelt chamber and the droplet maker may determine the range of melttemperatures accessible to the system. The material for the body 104 ofthe molten melt chamber 101 may be selected to be resistant to thetemperatures associated with the current technology. For example,ceramics, metals, or alloys may be used, including stainless steel, orsteel coated with stainless steel, vanadium, titanium, and the like. Insome embodiments, the material selected for use of the body 104 is notreactive or chemically inert/not-corrosive to the feedstock precursormaterial. As a result of using a material that can not only withstandthe temperature supplied to melt the feedstock, but also a material thatis non-reactive with the feedstock material, purity of the feedstock canbe maintained.

Embodiments may additionally provide further heating within the dropletmaker and within lines between the melt chamber and the droplet maker(if any), sufficient to maintain the molten state of the solid precursormaterial throughout the formation of the droplets.

The Droplet Maker

Droplet maker 110 depicts an embodiment that includes reservoir 114,actuating unit 111, surface 112, and nozzle 115. Actuating unit 111 maybe any actuating unit capable of achieving a uniform perturbation withinreservoir 114 such as, e.g., a piezoelectric actuator, a piston-drivenactuator, or an acoustic actuator. Surface 112 forms a portion ofreservoir 114 and communicates the action of actuating unit 111 to thecontents of reservoir 114. Molten transport line 106 feeds reservoir 114and, during operation, maintains reservoir 114 substantially full.Nozzle 115 may be any nozzle, orifice, slit, grating, capillary, tube,channel, or aperture in communication with or a feature of reservoir 114through which material within reservoir 114 may be released as droplets119 by the action of actuating unit 111. In some embodiment, the dropletmaker includes more than one, distinct and separate nozzles to createmultiple droplet flows. The droplet maker may be any suitable dropletmaker, such as any one of the droplet makers described in publicationsincorporated by reference herein.

Droplets 119 are of uniform diameter if the wavelength of the uniformperturbation pulses created by actuating unit 111, satisfy jet streambreak up according to Webber's law for viscous fluids:λ=√{square root over (2)}πd _(j)√1+3_(η)/√{square root over (ρσd _(j))}where d_(j) is the jet diameter, η is the fluid viscosity, ρ is thefluid density, σ is the surface tension, and λ is the wavelength of theperturbation pulses. The droplets produced are uniform and theirdiameter, d_(d), is approximately 1.89 times that of the jet diameter,d_(j).

In embodiments, surface 112 and/or reservoir 114 may be constructed of amaterial having low thermal conductivity in order to decrease theheating of actuating unit 111 or other components of droplet maker 110by the high temperature molten material within reservoir 114.Alternatively, actuating unit 111 may be in communication with, butlocated remotely from, surface 112 and/or reservoir 114 to furtherreduce heating of the actuating unit 111 or other components. Forexample, the communication between actuating unit 111 and surface 112and/or reservoir 114 may be provided by some length of rigid materialattached to both actuating unit 111 and surface 112 and having lowthermal conductivity, such that the rigid material communicates theaction of actuating unit 111 to surface 112 while limiting the transferof heat from surface 112 to actuating unit 111.

In embodiments where the actuating unit is a piezoelectric actuator, thepiezo is driven at a resonant frequency to produce a repeatabledeflection of the piezo. The piezo can be maintained operatively engagedwith the plate using, for example, a screw, or swivel bolt, that causesthe piezo to press against the plate. As the piezo is maintained incontact with the plate, the deflection of the piezo is transmitted tothe rigid plate. In turn, this deflection of the piezo, which can beless than about 5.0 micrometers, is transmitted to the fluid inside thereservoir to break down a jet of fluid exiting the capillary channels.The piezo actuator produces a perturbation that dissipates radially andlongitudinally. Thus, the farther from the center of the piezo actuator,the weaker the perturbation is. As will be appreciated, the size andgeometry of the chamber is correlated to the size and power of the piezoactuator, the material and properties of the plate, and the viscosity ofthe fluid in the chamber.

The reservoir 114 is made of a relatively corrosion resistant materialthat can maintain is mechanical strength at temperatures up to about1100° C., such as ceramics, stainless steel, or steel coated withstainless steel, vanadium, titanium, and the like. As a result, thereservoir 114 can contain the molten precursor materials.

Where actuating unit 111 may be selected in order to achieve uniformperturbation pulses, droplets 119 are expected to be uniform indiameter. In one embodiment, the droplet maker achieves substantiallyuniformity between droplets with a variation in droplet diameter of lessthan about 20%. For example, the variation of droplet size for dropletswithin a stream of droplets is within 20% (e.g., ±10% from apredetermined droplet size), within 10% (e.g., ±5% from a predetermineddroplet size), within 5%, within 4%, within 3%. While not wishing to bebound by theory, it is believed that the droplet diameter uniformity isdetermined by the accuracy of the perturbation wavelength (λ) in theabove equation where all other parameters are constant. Thus, bytailoring equipment and control of environment and materials used in thedroplet maker, better control of uniformity can be achieved as shown inWebber's law provided above.

The molten droplets formed by droplet maker 110, droplets 119, may thenbe released into plasma torch 121. In embodiments, the droplet maker andthe melt chamber may be combined in a single unit; that is, the solidmaterial may be loaded directly into the single unit, melted therein,and the droplets released therefrom.

The Plasma Torch

FIG. 3 depicts an example of a plasma torch which may be used withinembodiments of the present technology. In FIG. 3 , droplets 119 areshown entering plasma torch 121 along line 218 within tube 330. Droplets119 may be droplets produced by the droplet maker described herein, ormay be the product of a first plasma torch in those embodiments havingmore than one plasma torch.

Plasma torch 121 also includes microwave source 305 (i.e., an energysource for forming the plasma), reflected power protector 310, impedancematching unit 315, and waveguide 320. Waveguide 320 guides microwaveradiation 325 from microwave source 305 to plasma chamber 122 whichintersects waveguide 320. Waveguide 320 is a closed structure designedso that the microwave radiation exhibits peak intensity within plasmachamber 122.

Plasma torch 121 includes three concentric tubes, which are tubes 330,340, and 350. Tube 330 carries droplets 119. Tube 340 carries anentrainment gas flow 345 supplied by entrainment gas source 342. Tube350 carries a shrouding gas flow 355 supplied by shrouding gas source352.

Microwave source 305 may provide microwave radiation having frequenciesbetween about 500 MHz and about 100 GHz. More particularly, the rangemay be from about 900 MHz to about 5900 MHz. For example, one embodimentof a microwave source operated at 915 MHz and 75 kW. Another embodimentof a microwave source operated at 2.45 GHz. In other embodiments notshown, the energy source need not be a microwave energy source.Alternatively, the energy source could be a radio frequency radiationenergy source, or other electromagnetic energy sources.

Impedance matching unit 315 communicates with the microwave source 305and the waveguide 320 to minimize reflection of the microwave radiation325 from the plasma chamber 122 to the microwave source 305. The plasmatorch 121 can also include a reflected power protector 310 incommunication with the waveguide 320. The reflected power protector 310could be a waveguide circulator that deflects reflected portions of themicrowave radiation 325 to a dump receiver (not shown). The reflectedpower protector 310 alternatively could be a waveguide isolator.

During operation, entrainment gas flow 345 will entrain droplets 119within itself as it flows into plasma chamber 122. Within plasma chamber122, microwave radiation 325 will ignite entrainment gas 345 to formplasma 125. Plasma 125 will continue to be supplied by entrainment gasflow 345 including entrained droplets 119, if any. Shrouding gas flow355 may flow along wall 362 of plasma chamber 122 (which may be thecontinuation of tube 350 within plasma chamber 122) between wall 362 andplasma 125. Within plasma chamber 122, shrouding gas flow 255 providesseparation between plasma 125 and wall 362 of plasma chamber 122,protecting plasma chamber 122 from the extreme temperatures of plasma125.

A range of suitable gases is known for plasma sources. A nonreactiveplasma, such as an inert gas such as helium, argon, neon, xenon, kryptonor combinations of the same may be used. Alternatively, the plasma mayinclude a reactive chemical species which is to be incorporated into thequantum dot produce. For example, nitrogen or oxygen may be included inthe plasma source where a nitride or oxide quantum dot is desired (e.g.,TiN quantum dots, GaN quantum dots, AlGaN quantum dots, ZnO quantumdots, etc.). The shrouding gas flow may also be a flow of inert orrelatively gas such as nitrogen, helium, argon, neon, xenon, krypton, orcombinations of the same.

An advantage of the present technology is that microwave excitation ofthe plasma enables the microwave plasma torch to provide plasmatemperatures as high as about six thousand Kelvin (6000 K), nearly twicethe highest-known attainable stoichiometric combustion temperatures(three thousand Kelvin (3000 K) for a hydrogen-oxygen flame). Thus, thepresent technology can achieve thorough heating of a material in aninert, oxidizing or reducing environment, which typically cannot beattained using an oxy-fuel torch. In addition, the plasmas created bythe microwave plasma torch in accordance with an embodiment of thepresent technology have a substantially uniform radial temperatureprofile. Without wishing to be bound by theory, it is believed that theuniformity of the temperature profile in combination with the uniformityof the droplet size generated from the droplet maker together provide apreferred environment for uniform vaporization of the precursors. As aresult, preferred conditions arise from the substantially uniform andconsistent vaporization of individual droplets to create an advantageousenvironment for nucleation and crystallization of nanoparticles withinthe plasma chamber downstream of the plasma.

Another advantage of the present technology is that microwave excitationof the plasma jet efficiently converts more than about ninety percent(90%) of supplied microwave energy to enthalpy of the plasma jet. Thisefficiency exceeds that of other devices by a factor of between abouttwo times and about three times. Another advantage of the presentinvention is that multiple microwave plasma apparatuses can be stackedin a modular fashion because the waveguide, the impedance matching unit,and the reflected power protector cooperate to prevent interferencebetween adjacent plasma jets. Stacking multiple microwave plasmaapparatuses can permit extremely uniform heating of the process materialto temperatures between about three thousand Kelvin (3000 K) and aboutsix thousand Kelvin (6000 K). Of particular interest, stacked microwaveplasma apparatuses may be arrayed to receive within each plasmasuccessive plasma products created by the previous plasma, and plasmasmay each be at different temperatures. This arrangement permits, forexample, a first plasma to be used to create a quantum dot core from acore precursor while a second plasma may receive the quantum dot coreand a shell precursor and produce a core covered in a shell. In thisexample, the second plasma may be operated at a lower temperature toreduce or eliminate re-melting of the core within the second plasmawhile still vaporizing the shell precursor. An embodiment of a systemwith two plasma torches is shown in FIG. 4 . As shown, system 400includes a first molten melt chamber 401, a first droplet maker 410, afirst plasma torch 421 with a first plasma 425, a first reaction chamber430 and a first quenching chamber 435. The products of quenching chamber425, which may be quantum dot cores, enter chamber 480. Also, dropletsprepared by second droplet maker 460 additionally enter second plasmatorch 471 including second plasma 475, enter chamber 480. Downstream ofsecond quenching chamber 485 second quenching chamber 485 and filtersystem 495.

While not wishing to be bound by theory, it is believed that within theplasma flowstream, the droplets of liquid rapidly vaporize, but theatoms thereof remain clustered to some degree and continue to passthrough the plasma and into reactor 130, the portion of plasma torch 121downstream of plasma 125. Within reactor 130, the temperature issufficiently low, relative to plasma 125 and to the evaporation point ofthe precursor materials, that the gaseous material begins to condenseand nucleation begins (i.e., in some embodiments, quenching andnucleation begin in the plasma chamber). The condensing and nucleatingmaterial continues to pass through the system at a rate which iscontrolled to allow for crystallization of the material. That is, rateof cooling is controlled within the reactor (i.e., the portiondownstream of the plasma) to the extent that crystallization (withlimited growth) can occur. As the particles transition into quenchingchamber 135 through perforated surface 132, the combination of coolingand expansion is believed to quench the nanoparticle growth. As notedabove, in some embodiments, an advantageous environment for producingquantum particles (e.g., QD) arises from the combination ofsubstantially uniform droplets of precursor being delivered to a plasmawith a substantially uniform radial temperature profile. Theseconditions create an advantageous or optimal environment for thenucleation and crystallization of nanoparticles. To lock thecrystallized structure and size of the particle in place, a quenchingstep is utilized.

The Quenching Chamber

Embodiments of the quenching chamber may have various optional featuresto increase the quenching rate. Increased quenching rate may beparticularly important when manufacturing quantum dots which growrapidly or when a particular size is desired, or more particularly asmall size. Quenching may be increased by introducing low pressure, lowtemperature, or both within the quenching chamber. For example, thepressure proximate the plasma may be greater than atmospheric pressure,such as about 14 to about 20 psi. By the use of a vacuum pump, apressure below atmospheric pressure, such as a pressure of about 1 toabout 100 Torr may be used within the quenching chamber. The use of alower pressure encourages expansion of the gas and reduce quantum dotgrowth. The pressure may be varied, for example, to change the rate ofquenching. The pressure may be about 5 Torr, about 10 Torr, about 20Torr, about 30 Torr, about 40 Torr, about 50 Torr, about 60 Torr, about70 Torr, about 80 Torr, about 90 Torr, or about 100 Torr. These valuesmay also be used to define a range, such as from about 30 Torr to about80 Torr. Additionally, lower temperature may be provided within thequenching chamber. For example, the quenching chamber may be outfittedwith an active cooling means. The quenching chamber may be provided witha cooling fluid flow, such as a flow of high pressure helium through thevessel. In an embodiment, the cooling fluid flow may be introduced intothe quenching chamber from the side of the chamber opposite the plasma,such that as the particles enter the chamber from the plasma torch, acooling flow may be directed at the particle flow from the opposingdirection. Preferably, the cooling fluid flow will be arranged so thatthe flow is introduced uniformly with respect to the particle flow sothat substantially all of the particles experience a similar coolingprofile, in order to encourage more uniform growth and quenching amongthe population. For example, by introducing the cooling fluid oppositethe plasma chamber, rather than across the quenching chamberperpendicularly to the flow, all portions of the material will interactwith the cooling fluid under approximately the same conditions.

An embodiment of a quenching chamber exhibiting a cooling feature may beobserved in FIG. 2 , where quenching chamber 135 receives particle flowfrom the reaction chamber 130 portion of plasma torch 121 throughperforated surface 132, which has a large number of apertures throughwhich material may pass. Additionally, quenching chamber 135 includesmanifold 137 along the side of quenching chamber 135 opposite perforatedsurface 132. Manifold 137 may be operated with vacuum pump 139 in orderto maintain low pressure within quenching chamber 135. In order toassist in maintaining the pressure gradient, perforated surface 132 maymore preferably be replaced with a series of nozzles to better separatethe high and low pressure portions and permit the quantum dot materialto flow into the chamber without excessive quantity of gas.Alternatively, manifold 137 may be fed a stream of cooling gas bycooling gas source 138 in order to provide a reduced temperature withinquenching chamber 135. In some embodiments, not shown a piece of copperseverely cooled (e.g., cooled with liquid nitrogen or liquid helium) canbe used to quench the forming quantum particles. That is, the severelycooled copper plate provides a temperature differential to thenucleating/crystallizing particles passing from the plasma chamber andonto the copper plate. As a result of this extreme differential, growthof the particle is stunted and uniform quantum particles are formed.

The quenching chamber may use chemical, as well as physical (i.e.,temperature, pressure, etc.) methods to achieve and encourage quenching.For example, a capping or functionalizing chemical species may beintroduced into the quenching chamber so that the capping species reactswith the quantum dots so as to quench the growth of the quantum dots. Insome cases, these chemical species may be introduced onto the outersurface of the quantum dot, such as organic moiety including amines,pyridines, carboxyl groups, alkyl groups, etc. Turning to FIG. 2 ,quenching chamber 135 also includes chemical source 136 which may supplya flow of such compounds through manifold 137 into quenching chamber135. In other embodiments, a quenching chamber may be provided with morethan one introduction point for a functionalizing species so that aseries of reactions may be achieved. For example, a shell material maybe introduced in one step and subsequently a functionalizing species maybe introduced to form a layer of that functional moiety on the shell.

It may be appreciated that in addition to quenching growth of thequantum dot, the functional groups introduced onto the surface mayadditionally provide desirable functionalities to the quantum dot. Thespecies may prevent agglomeration of the particles, may imparthydrophobicity or hydrophilicity, may provide a portion of a linkage tobe subsequently attached to another particle, compound, or substrate,may add desirable electronic or optical characteristics, and so on.

These quenched nanoparticles are then transported to filter system 145along line 140. Filter system 145 is used to collect the nanoparticles.

As discussed above, uniformity of particle size in a manufactured batchof quantum dots is desirable. Several characteristics may be used toquantify that uniformity. For example, size of the particles may bedetermined by direct measurement, such as using transmission electronmicroscopy (TEM). Alternatively, a particle size distribution may bedetermined indirectly. For example, methods are known for estimating thedistribution based upon the characteristics of an ultraviolet-visiblespectroscopy spectrum (such as by measuring the full width half max(FWHM) of a peak associated with the quantum dot).

Without wishing to be bound by theory, the present technology achievesuniformity of particle size within a manufacturing run due to uniformityand consistency provided by the novel manufacturing route. Inparticular, by forming and providing small (i.e., 100 micron or less),substantially uniformly sized (i.e., within 50% of a predetermined size)flow of droplets formed from a molten homogeneous material to a plasmawith a uniform radial temperature profile, vaporization of the flow ofdroplets is both uniform and consistent. As a result, controlled coolingor quenching of a uniformly formed vapor allows for manufacture ofuniformly sized quantum particles. Collection techniques may, in someembodiments, include functionalizing techniques to preventagglomeration. Additionally or alternatively, collection techniques ofthe present technology can include filtering and additional processingto create tailored structures.

EXAMPLES

The following examples illustrate possible embodiments of the presenttechnology. These examples are not limiting.

Example 1 (Formation of a Zinc Oxide Quantum Dot)

Using the system shown in FIG. 2 , zinc oxide quantum dots are formedusing the following procedure:

-   -   (1) Add solid zinc metal to body 104 of molten melt chamber 101.    -   (2) Purge melt chamber 101 by drawing off released contaminant        gases through vent 109, and purge molten transport line 106 and        reservoir 114 with an inert gas.    -   (3) Melt the solid zinc within molten melt chamber 101 using        heating element 105 by heating to above the melting point of        zinc (419.5° C.).    -   (4) Apply a pressure of gas from layer of pressurized gas 103 by        means of gas supply 108 to force molten zinc precursor to flow        through molten transport line 106.    -   (5) Fill reservoir 114 of droplet maker 110 with molten zinc        precursor through molten transport line 106.    -   (6) Oscillate surface 112 at 12 kHz through the action of        actuating unit 111, which maybe a piezoelectric actuator,        achieving a stream of droplets 119 from droplet maker 110.    -   (7) Establish an oxygen based plasma (e.g., oxygen,        oxygen-helium, or oxygen-argon) as plasma 125 within plasma        chamber 122 of plasma torch 121.    -   (8) Permit stream of droplets 119 to enter plasma 125.    -   (9) Permit the action of plasma 125 to vaporize the        substantially molten zinc precursor of droplets 119.    -   (10) Permit quantum particle growth from the zinc precursor        vapor as the vapor travels into reactor 130.    -   (11) Permit the forming quantum particles to enter quenching        chamber 135 through perforated surface 132 (to lock in crystal        structure and inhibit further growth).    -   (12) Subject the forming quantum particles to a flow of helium        cooling gas from cooling gas source 138 through manifold 137.    -   (13) Permit the conditions of quenching chamber 135 to quench        growth of the quantum particles.    -   (14) Collect quantum particles within filter system 145.

Example 2 (Formation of a Tin-Silicon Quantum Dot)

Using the system shown in FIG. 2 (with optional features shown in FIG. 3), tin-silicon quantum dots are formed using the following procedure:

-   -   (1) Add solid tin metal to body 104 of molten melt chamber 101.    -   (2) Purge melt chamber 101 by drawing off released contaminant        gases through vent 109, and purge molten transport line 106 and        reservoir 114 with an inert gas.    -   (3) Melt the solid tin within molten melt chamber 101 using        heating element 105 by heating to above the melting point of tin        (232° C.).    -   (4) Apply a pressure of gas from layer of pressurized gas 103 by        means of gas supply 108 to force molten tin precursor to flow        through molten transport line 106.    -   (5) Fill reservoir 114 of droplet maker 110 with molten tin        precursor through molten transport line 106.    -   (6) Oscillate surface 112 at 12 kHz through the action of        actuating unit 111, achieving a stream of droplets 119 from        droplet maker 110.    -   (7) Establish an argon plasma (e.g., argon, argon/helium) as        plasma 125 within plasma chamber 122 of plasma torch 121.    -   (8) Flow silane gas (SiH₄) through tube 340 into plasma chamber        122.    -   (9) Permit stream of droplets 119 to enter plasma 125.    -   (10) Permit the action of plasma 125 to vaporize the tin        precursor of droplets 119.    -   (11) Permit quantum dot nucleation and growth from the molten        tin precursor vapor and from the silane gas as the vapor travels        into reactor 130.    -   (12) Permit the forming quantum dot to enter quenching chamber        135 through perforated surface 132.    -   (13) Submit the forming quantum dot to a flow of helium cooling        gas from cooling gas source 138 through manifold 137.    -   (14) Permit the conditions of quenching chamber 135 to quench        growth of the quantum dots.    -   (15) Collect quantum dots within filter system 145.

Example 3 (Formation of a Cadmium-Telluride Quantum Dot)

Using the system shown in FIG. 2 cadmium-telluride quantum dots areformed using the following procedure

-   -   (1) Add solid cadmium and tellurium metals to body 104 of molten        melt chamber 101.    -   (2) Purge melt chamber 101 by drawing off released contaminant        gases through vent 109, and purge molten transport line 106 and        reservoir 114 with an inert gas.    -   (3) Melt the solid cadmium and tellurium metals within molten        melt chamber 101 using heating element 105 by heating to above        the melting point of both cadmium (321° C.) and tellurium        (449.5° C.).    -   (4) Mix the molten cadmium and tellurium to achieve a thoroughly        mixed molten mixture of cadmium and tellurium.    -   (5) Apply a pressure of gas from layer of pressurized gas 103 by        means of gas supply 108 to force molten precursor mixture to        flow through molten transport line 106.    -   (6) Fill reservoir 114 of droplet maker 110 with molten mixture        precursor through molten transport line 106.    -   (7) Oscillate surface 112 at 12 kHz through the action of        actuating unit 111, achieving a stream of droplets 119 from        droplet maker 110.    -   (8) Establish an argon or argon/helium plasma as plasma 125        within plasma chamber 122 of plasma torch 121.    -   (9) Permit stream of droplets 119 to enter plasma 125.    -   (10) Permit the action of plasma 125 to vaporize the        substantially molten mixture precursor of droplets 119 to form a        vapor.    -   (11) Permit quantum dot nucleation and growth from the vapor as        the vapor travels into reactor 130.    -   (12) Permit the forming quantum dots to enter quenching chamber        135 through perforated surface 132.    -   (13) Subject the forming quantum dots to a flow of helium        cooling gas from cooling gas source 138 through manifold 137.    -   (14) Permit the conditions of quenching chamber 135 to quench        growth of the quantum dots.    -   (15) Collect quantum dots within filter system 145.

Example 4 (Formation of a Cadmium-Selenium (Core)/Zinc-Sulfide (Shell)Quantum Dot)

Using the system shown in FIG. 4 cadmium-selenium/zinc-sulfidecore/shell quantum dots are formed using the following procedure:

-   -   (1) Add solid cadmium and selenium to molten melt chamber 401.    -   (2) Purge melt chamber 401, molten transport line 406 and        reservoir 414 with an inert gas.    -   (3) Melt the solid cadmium and selenium within molten melt        chamber 401 by heating to above the melting point of both (321°        C.) and selenium (220.8° C.).    -   (4) Mix the molten cadmium and selenium to achieve a thoroughly        mixed mixture of cadmium and selenium.    -   (4) Provide the molten mixture to droplet maker 410.    -   (5) Provide a stream of droplets 436 from droplet maker 410 into        plasma torch 421.    -   (6) Establish an argon plasma as plasma 425 in plasma torch 421.    -   (7) Permit the action of plasma 425 to vaporize the molten        mixture precursor droplets.    -   (8) Permit quantum dot core growth from the molten mixture        precursor vapor as the vapor travels into reactor 430.    -   (9) Permit the vapor to enter quenching chamber 435.    -   (10) Permit the conditions of quenching chamber 435 to quench        growth of the quantum dot cores.    -   (11) Add solid zinc and sulfur to second molten melt chamber        451.    -   (12) Melt the solid zinc and sulfur within second molten melt        chamber 451 by heating to above the melting point of both zinc        (419.5° C.) and sulfur (115.2° C.).    -   (13) Mix the molten zinc and sulfur to achieve a thoroughly        mixed mixture of zinc and sulfur.    -   (14) Supply the molten mixture to second droplet maker 460.    -   (15) Provide a stream of molten mixture droplets 461 from second        droplet maker 460 into a second plasma torch 471.    -   (16) Establish an argon plasma as second plasma 475 within        second plasma torch 471.    -   (17) Permit the action of second plasma 475 to vaporize the        molten mixture precursor of droplets 461 of zinc and sulfur.    -   (18) Pass the vaporized molten mixture created in step (17) into        chamber 480.    -   (19) Introduce the core quantum dots produced in step (10) into        chamber 480 to permit a zinc sulfur shell to form over the        cadmium-selenium quantum dot cores within reaction chamber 480.    -   (20) Permit the material to enter second quenching chamber 485.    -   (21) Permit the conditions of second quenching chamber 485 to        quench growth of the quantum dot shells.    -   (22) Collect cadmium-selenium (core)/zinc-sulfur (shell) quantum        dots within filter system 495.

Example 5 (Formation of a Functionalized Cadmium-Telluride Quantum Dot)

Using the system shown in FIG. 2 cadmium-telluride quantum dotsfunctionalized with a carboxylic acid group are formed using thefollowing procedure

-   -   (1) Add solid cadmium and tellurium metals to body 104 of molten        melt chamber 101.    -   (2) Purge melt chamber 101 by drawing off released contaminant        gases through vent 109, and purge molten transport line 106 and        reservoir 114 with an inert gas.    -   (3) Melt the solid cadmium and tellurium within molten melt        chamber 101 using heating element 105 by heating to above the        melting point of both cadmium (321° C.) and tellurium (449.5°        C.).    -   (4) Mix the molten cadmium and tellurium to achieve a        homogeneous mixture of cadmium and tellurium.    -   (5) Purge melt chamber 101 by drawing off released contaminant        gases through vent 109.    -   (6) Apply a pressure of gas from layer of pressurized gas 103 by        means of gas supply 108 to force molten precursor mixture to        flow through molten transport line 106.    -   (7) Fill reservoir 114 of droplet maker 110 with molten mixture        precursor through molten transport line 106.    -   (8) Oscillate surface 112 at 12 kHz through the action of        actuating unit 111, achieving a stream of droplets 119 from        droplet maker 110.    -   (9) Establish an argon plasma or argon/helium plasma as plasma        125 within plasma chamber 122 of plasma torch 121.    -   (10) Permit stream of droplets 119 to enter plasma 125.    -   (11) Permit the action of plasma 125 to vaporize the molten        mixture precursor of droplets 119 to form a vapor.    -   (12) Permit quantum dot nucleation and growth from the precursor        vapor as the vapor travels into reactor 130.    -   (13) Permit the forming quantum dots to enter quenching chamber        135 through perforated surface 132.    -   (14) Subject the forming quantum dots to a gas flow including        vaporized 3-mercaptopropionic acid from chemical source 136        through manifold 137.    -   (15) Permit the conditions of quenching chamber 135 to quench        growth of the quantum dots, such that the thiol portion of the        3-mercaptopropionic acid bonds to the exterior surface of the        cadmium telluride quantum dot.    -   (16) Collect the cadmium telluride quantum dots functionalized        with a carboxylic acid group within filter system 145.

While the present disclosure has been described with reference toparticular embodiments, it will be understood by those skilled in theart that various changes may be made and equivalents may be substitutedfor the elements thereof without departing from the scope of thedisclosure. In addition, many modifications may be made to adapt theteaching of the present disclosure to particular use, application,manufacturing conditions, use conditions, composition, medium, size,and/or materials without departing from the essential scope thereof.Therefore, it is intended that the present disclosure not be limited tothe particular embodiments and best modes contemplated for carrying outthis disclosure as described herein. The accompanying claims areintended to cover such modifications as would fall within the true scopeand spirit of the present disclosure.

What is claimed is:
 1. A method of manufacturing core-shell particles bya microwave plasma process, the core-shell particles having a core atleast partially surrounded by a shell, wherein core material and shellmaterial are chemically distinct; the method comprising: providing amicrowave plasma reactor at least having a plasma chamber; introducing agas or vapor to the plasma chamber; generating a plasma with a microwaveenergy source within the plasma chamber using the gas or vapor to createa microwave formed plasma; injecting core precursor material into themicrowave formed plasma within the plasma chamber; vaporizing of thecore precursor material within the plasma chamber by the microwaveformed plasma; forming a plurality of cores from the vaporized coreprecursor material; introducing shell precursor material to theplurality of cores; and forming a plurality of core-shell particles,each core-shell particle having one core at least partially surroundedby shell material; wherein the plurality of core-shell particlescomprises a cadmium containing core and a zinc containing shell.
 2. Themethod of claim 1, wherein forming a plurality of cores from thevaporized core precursor material comprises quenching the vaporized coreprecursor material.
 3. The method of claim 2, wherein quenching thevaporized core precursor material occurs in a quenching chambercomprising one or more exterior walls and a cooling fluid source, atleast one exterior wall comprising a plurality of holes, the pluralityof holes disposed to receive a flow of cooling fluid from the coolingsource.
 4. The method of claim 1, wherein forming a plurality ofcore-shell particles comprises quenching the plurality of cores afterintroduction of shell precursor material.
 5. The method of claim 4,wherein quenching the plurality of cores after introduction of shellprecursor material comprises introducing a cooling fluid.
 6. The methodof claim 1, wherein introducing shell precursor material comprisesintroducing a fluid to the plurality of cores.
 7. The method of claim 6,wherein the fluid includes the shell precursor material.
 8. The methodof claim 6, wherein the fluid reacts with the plurality of cores to formthe shell.
 9. The method of claim 6, wherein the fluid is a solution.10. The method of claim 6, wherein the fluid is a gas.
 11. A method ofmanufacturing core-shell particles having a core at least partiallysurrounded by a shell, wherein core material and shell material arechemically distinct; the method comprising: providing a core precursormaterial to a material feeding system, the core precursor material beingin the form of particles; introducing a gas or vapor into a first plasmachamber; generating a plasma in the first plasma chamber using a firstenergy source, the first energy source in communication with the firstplasma chamber and disposed to apply energy to the gas or vapor in thefirst plasma chamber to form the plasma; injecting the core precursormaterial from the material feeding system into the plasma; vaporizingthe core precursor material to form a plurality of cores from theinjected core precursor material; cooling the plurality of coresdownstream of the first plasma chamber; and introducing a shellprecursor material to the plurality of cores after cooling to form aplurality of core-shell particles, each core-shell particle having ashell covered core.
 12. The method of claim 11, wherein introducing ashell precursor material comprises injecting the shell precursormaterial into a second plasma chamber, the second plasma chamberdisposed to receive the plurality of cores downstream of the firstplasma chamber.
 13. The method of claim 12, further comprisinggenerating a microwave plasma in the second plasma chamber.
 14. Themethod of claim 13, further comprising quenching of the plurality ofproduct particles downstream of the second plasma chamber.
 15. Themethod of claim 11, wherein the first energy source comprises amicrowave energy source having a radiation frequency from about 900 MHzto about 5900 MHz.
 16. The method of claim 11, wherein the materialfeeding system is heated.
 17. The method of claim 11, wherein the shellprecursor material comprises a fluid.
 18. The method of claim 11,wherein the core precursor material comprises cadmium.
 19. The method ofclaim 11, wherein the shell precursor material comprises zinc.